This application claims the benefit under 35 U.S.C. 119(a) of European patent application No. 200951.1 filed May 3, 2002.
1. Field of the Invention
The present invention generally refers to the field of digital to analogue conversion, and more particularly to thermometer code based digital to audio conversion.
2. Background and Prior Art
Thermometer code digital to audio converters DACs are as such known from the prior art. Conventional thermometer codes DACs are of the so-called current-steering type. For converting of an m-bit digital input signal a conventional DAC of the current-steering type includes a plurality of n identical current sources, where n=2mxe2x88x921. Each of the current sources passes a substantially constant current I.
Each of the current sources is switchable in order to perform the digital to analogue conversion. In such a conventional DAC the switching of the current sources I is controlled by thermometer-coded signals. For an overview of the respective prior art reference is made to U.S. Pat. No. 6,163,283.
U.S. Pat. No. 6,359,467 shows a DAC which converts a randomised digital code into an analogue signal. The randomisation is performed by a current-mode randomiser which randomises a digital code based on a control word provided by a pseudo random number generator.
U.S. Pat. No. 6,225,929 shows a DAC having switchable current sources and a resistor string. The resistor string includes N resistors serially coupled between a ground node and the analogue voltage output signal. Nodes 1 through Nxe2x88x921 are defined at junctions between the N resistors and a node N is coupled to the analogue voltage output signal. Each of N switchable current sources is controlled by a corresponding one of the N bits of the digital input signal to supply current to a corresponding one of the N nodes when its corresponding one of the N bits is in the first state and to not supply current to its corresponding one of the N nodes when its corresponding one of the N bits is in the second state. The DAC includes thermometer converting logic for controlling each switchable current source.
In general each current source is enabled for a different amount of time and that mismatches between the implemented currents have an increased effect.
It is an object of the present invention to provide an improved method for digital to analogue conversion and an improved DAC, in particular for audio applications.
The object of the present invention is solved basically by applying the features laid down in the independent claims. Preferred embodiments of the invention are given in the dependent claims.
The invention provides for an efficient method and apparatus to reduce the effects of current source mismatch on signal distortion in the digital to analogue conversion process. In essence, this is accomplished by controlling the operation of the randomiser based on the digital signal to be converted itself.
In accordance with a preferred embodiment of the invention the randomiser comprises a barrel shifter for randomisation of the thermometer-coded signal. Preferably the barrel shifter is controlled by means of a signal that is derived from the digital signal to be converted.
In accordance with a further preferred embodiment of the invention the digital signal to be converted has a width of mbits. A modulo n of the digital signal is determined, where n=2mxe2x88x921. The modulo n of the digital signal can be used as a control signal for the barrel shifter.
In accordance with a further preferred embodiment of the invention a noise shaper is used to shift quantization noise to higher frequencies. The output of the noise shaper is converted to a thermometer-coded signal. The operation of the randomiser is controlled by the output signal of the noise shaper. This has the advantage that the quantization noise is not shifted back into the audible spectrum as this can be the case in prior art randomisers.
In accordance with a further preferred embodiment of the invention the noise shaper has a control circuit for resetting the noise shaper when the input signal remains at a predetermined signal level for a predetermined period of time in order to suppress undesired signal output of the noise shaper during pauses, in particular for audio applications.
The present invention is particularly advantageous in that it enables to substantially reduce the signal distortion of a DAC which are caused by tolerances of the current sources. Further the present invention enables to eliminate audible noise which is produced during pauses. This way the audio quality is substantially improved.